1. Field of the Invention
The present invention concerns an electrochemical generator operating reversibly at ambient temperature, the electrolyte of which is a solid solution of an ionic compound in a macromolecular material, which generator is distinguished by the particular nature of the active material of its positive electrode.
2. Discussion of the Background
European patent No. 13199 has already proposed electrochemical generators of the thin film type the electrolyte of which is a solid solution of an ionic compound in a macromolecular material. According to this patent, the positive electrode can be constituted by the agglomeration product, forming a composite mass of the electrochemically active material, of a ionic conduction inert compound and of a macromolecular material that is identical or similar to that constituting the electrolyte. In this cited European patent, the preferred material for producing the electrolyte is a poly(ethylene polyoxide). Numerous materials have been proposed as electrochemically active material. However, the choice of such materials is much more limited when it is desired to produce rechargeable generators that are able to provide high and stable voltages through the charge and discharge cycles.
In order to obtain generators operating at ambient temperature, it is preferable to select a macromolecular material constituted by an ethylene oxide copolymer and a cyclic ether oxide, substituted or not, presenting a polyether type chain. A material in the form of a single thermodynamically stable phase which facilitates correct operation of these generators at ambient temperature is thus obtained. In association with these electrolytes, a certain number of electrochemically active materials exist that can be utilized and which allow reversible operation at ambient temperature. For example, derivatives of transition elements such as molybdenum dioxide, vanadium oxide and titanium disulphide may be cited. These compounds present the common property of essentially giving rise to insertion reactions with the salt cation in solid solution within the electrolyte material.
The utilization of these materials, herein-after referred to as insertion materials since they essentially give rise to insertion reactions, present several drawbacks. Generally, they are costly materials the synthesis of which is difficult to carry out. Furthermore, the insertion reactions only involve about a single electron per atom of active material (between 1 and 1.5). Due to this fact, the theoretical capacity of the electrode is small. When the active material is, for example, vanadium oxide V.sub.6 O.sub.13, this capacity is about 420 Ah/kg of active material. When the active material is TiS.sub.2 this capacity is about 250 Ah/kg of active material.
It has, on the other hand, been proposed without great success to use various electrode materials other than insertion materials to produce secondary electrochemically active generators, but however these materials were associated with electrolytes that are very different from those concerned by the generators according to the present invention, such as, for example, molten salts or organic solvents. Until now, the best example proposed is that of a generator utilizing an iron pyrite as the active material of an electrode, this electrode being associated to an organic electrolyte operating at ambient temperature (AMBIENT TEMPERATURE SECONDARY Li/FeS.sub.2 CELLS G. H. NEWMAN; L. P. KLEMANN, Proceedings of the 29th Power Sources Conference, June 1980). According to this article, an attempt had been made to produce a rechargeable generator operating at ambient temperature. The authors of this article tested a whole series of organic electrolytes such as, or example, dioxolanedimethoxyethane mixture comprising in solution a lithium organo-borate, the aim of these tests being to locate an electrolyte couple of electrode material allowing good cycling of the generator at ambient temperature. By referring to the above-mentioned article, it can be seen that these tests were not favorable since the best cycling performance realized only allowed to achieve 22 cycles with discharge capacities that considerably decreased as the number of cycles was progressively increased. For example, an initial value of 128 mA-hours for the first cycle decreased to 71 mA-hours for the 22nd cycle, which illustrates a sharp decrease in capacity. The authors of the above article thus concluded that they were not yet able to carry out reversible operating of a generator over a great number of cycles, at ambient temperature, when the electrode material is FeS.sub.2, i.e. a material giving rise, during its reduction, to the nucleation of new species. For a fuller explanation of the different reactions involved by this reduction, reference may be made, for example, to the article of M. B. CLARK LITHIUM BATTERIES, Edited by J. M. GABANO 1983, ACADEMIC PRESS.
Without wishing to advance a limitative scientific theory, the authors of the present invention feel able to explain this impossibility of using FeS.sub.2 as the electrode material of a reversible liquid electrolyte accumulator at ambient temperature by the fact that the transport mechanisms of these electrolytes allow novel species generated during reduction to migrate or to diffuse due to the absence of selectivity; this absence of selectivity modifies the electrochemical processes or the availability of active materials during the discharge or charge of the generator.